Lecture 9 Flashcards
What are chalcogens?
The chalcogens are sulfur, selenium, and tellurium, the elements right below oxygen in the periodic table. There are chalcogenides analogous to most binary oxides, as well as some more complex oxide structures like perovskites and spinels. In these related materials, the chalcogen replaces the oxygen in the lattice.
What are the fascinating aspects of chalcogens?
Chalcogens have a richer chemistry than oxygen, exemplified by their oxidation states. Oxygen only shows a -2 state while chalcogens can have +4, +6 or -2. Their higher polarizability, due to their larger number of electrons, facilitates the formation of van der waals interaction in solids. This allows them to form semi-molecular solids in which covalent and van der Waals interactions are keeping the material together. The non-negligible effect of such interaction leads to chalcogenides with unique structures that find no parallel in the oxide world. A typical example is layered materials like MoS2.
The introduction of sulfur, selenium, or tellurium generally and gradually transforms the material into a semiconductor or a metal.
What is the main issue of chalcogenides?
The main issue of chalcogenides, when compared with oxides, is their sensitivity to oxidation and hydrolysis. This simple chemical aspect has made the advancement in the synthesis and characterization of their materials much slower than for their oxygen-based “cousins”.
Their synthesis and handling must usually be performed in oxygen-free conditions, especially when high temperatures are needed.
another aspect worth keeping in mind is when handling these materials is that their hydrolysis product is the corresponding acid (shown in digital notes). Where H2S, H2Se and H2Te are all gases of increasing toxicity.
What are quantum dots?
Nano-particles that are made of semiconducting materials (such as chalcogenides) and which exhibit electronic properties similar to those of atoms.
What is a core-shell architecture?
A structure that occurs when a nanoparticle has an outer coating that is a different material from the main particle.
Such structures have purposes ranging from multifunctionality to increased chemical stability, to enhanced photoluminescence, and more.
What are the challenges faced during the synthesis of core-shell architecture?
- Selectively grow the shell’s material with controlled thickness on top of the core material without incurring into homogenous nucleation (The shell material nucleating by itself);
- To grow the shell in a homogeneous way, uniformly protecting every part of the core surface;
- To maintain the colloidal stability of the core during shell growth.
What is the surface of CdSe like?
Due to chalcogenides being sensitive to water and oxygen, the surface of CdSe nanostructures needs to be separated from the outside world by a more water-resistant material, such as ZnS. This is an example of a core-shell architecture.
For CdSe the crystalline core forces us to use a shell material that has a similar structure and similarly sized unit cell. The reason for this lies in the energetics of the resulting solid-solid interface. On one hand, the growth of the shell material on top of the core surface will be promoted if the atomic structures match. If the lattices are different there will be a so-called structural mismatch, which will decrease the probability that a stable nucleus of the shell material will be able to form and grow uniformly over the core surface.
On the other hand, the two structures must also have very similar unit cell sizes, so that the lattice planes are as commensurate as possible; this is referred to as epitaxy. If this is not the case the growth of the shell material will lead to a strain of the lattice at the interface as the two different materials try to fit together as shown in Figure 5.1.
What are the two strategies to grow shells on CdSe nanocrystals?
- Heterogeneous nucleation scheme
- SILAR (successive ion layer adsorption and reaction)
Heterogeneous nucleation scheme
The technique is based on the reaction of diethylzinc and bis-trimethylsilyl sulfide in the presence of capped CdSe nanocrystals and ligands.
Such a reaction, if carried out properly, will lead to the formation of nuclei of ZnS on the surface of the CdSe nanocrystals. These shell nuclei will then grow to cover the whole surface of the core (Figure 5.1).
The disadvantage of this method is that it usually produces more than one nucleus on each nanocrystal leading to a polycrystalline shell of ZnS crystal grains. Grain boundaries are defects in the crystalline structure, and generally allow molecular diffusion through them: the consequence is that CdSe/ZnS nanocrystals grown in the way are often not as chemically stable as they could be, because of the possibility for water molecules, protons and oxygen to diffuse through the grain boundaries.
SILAR
A more elegant method. One layer of ions at a time is reacted with the surface of the core, in principle permitting the formation of a uniform coating of the core devoid of grain boundaries.
The CdSe cores are first reacted at high temperatures with stoichiometric amounts of zinc oleate, a slat formed by zinc cation and a fatty acid called oleic acid, a compound of olive oil. This reaction will lead to a CdSe core whose selenium atoms are coordinated by zinc cations forming a zinc-rich surface (Figure 5.1). A stoichiometric amount of sulfur in solution is then injected into the system, leading to the formation of a monolayer of ZnS, as the sulfur atoms react with the excess zinc ions on the surface deposited in the previous step.
This cycle can be repeated till the correct shell thickness is achieved. For most purposes, 3 to 4 layers are enough to protect the core to a sufficient degree, even though as many as 30 bilayers have been deposited.
The advantage is producing very stable nanocrystals with very predictable sizes and thus properties. The disadvantage is that every layer needs 1-3 hours to grow.
This technique has been refined to the point that it allows the formation of very sophisticated core-shell architectures such as CdSe/ZnS/CdSe or CdSe cores overgrown with shells of graded composition to help compensate for lattice mismatch problems.
Why is control the size of CdSe very important?
The reason for it being very important lies in the exquisite level of control of the electronic structure of this material that can be obtained by changing its size. The trigger for such activity was the development of the first route to the synthesis of low-polydispersity and colloidally stable CdSe nanocrystals.
What is the electronic structure of CdSe? (general recap of material science)
As we mentioned, CdSe is a semiconductor. This means that its electrons completely fill the valence band, which is separated from the conduction band by an energy bandgap (Figure 5.2).
CdSe, like any standard semiconductor, puts all of its electrons into available levels in the valence band. At higher energy, separated by the electronic bandgap, there is another continuum of levels called the conduction band in which all levels are virtually free.
(To help better understand this a slide is included in digital notes)
What is the mobility of electrons in the valence and conduction band like?
The mobility of electrons in the valance band is much lower than the of the conduction band. The reason for such low mobility is in the absence of available extra levels of comparable energy in which electrons can move; remember, all the valence states are taken. The only way the electron can move is to jump to the conduction band and that requires it to receive energy equal to the energy of the band gap.
How can electrons in semiconductors be brought to the conduction band?
- Thermal energy: The thermal energy is an amount of energy that is available to every particle and that represents their temperature. In semiconductors the Eg is larger than the thermal energy at room temp, thus very few electrons statistically get enough energy to move even briefly into the conduction band. Thus the conductivity is very low. Differently from metals, semiconductors conductivity increases with increasing temperature since the increasing thermal energy will knock an increasing number of electrons into the conduction band, where they can move and transport charge.
- Light: Photons are the basic constituents of light and carry a finite amount of energy, which depends on their frequency via the formula E=hv. If the energy of the impinging photons is equal to or larger than Eg, then an electron has a finite probability of absorbing it and being knocked into the conduction band.
What is the relation of the absorbance spectrum and Eg?
In Figure 5.2, we show how the absorbance spectrum of a semiconductor depends on the Eg. For energies higher than Eg, you start observing the absorption of light. this behaviour is the reason why semiconductors have colours which depend on their Eg.
For example: For instance, if Eg was 0.42 eV, as in PbS, the material would absorb all of the visible light, making PbS a black compound; if Eg was 3 eV, then the material would not absorb any visible light and would therefore be colourless
What happens to the nucleus when the electron roaming around it gets into the conduction band and moves away?
A positive charge, called a hole, is left behind. This positive charge will occupy the empty electronic level left in the valence band, a level which can be filled by neighboring electrons. The effect of a valence electron moving into a hole is the movement os the in its place. In this sense holes can be concidered as charge carries with an opposite sign to the electron; while electrons bring negaitve charhers, hole bring positive charge.
What does recombination of the hole and electron depend on?
we see in Figure 5.2 that the hole and the electron recombine in ways that depend on the material and its surface conditions. In some cases, the electron is trapped by surface defects where it can stay for relatively long times before relaxing to its hole and releasing energy in the form of heat. In other cases, relaxation happens directly, and the extra energy is released in the form of light, by the emission of a photon with energy equal to the energy of the exciton (slightly smaller than Eg)
What happens when you decrease the size of CdSe to the nanoscale?
The dynamics of an electron in an orbital such as the 1s ecitonic orbital is described as a wave function. A wave function describes the probability of finding the electron at any point in space. Every wave can be described in terms of wavelength so you can imagine our electron in the excitonic 1s orbital to be represented as a wave with a certain wavelength. For our purposes, such a wavelength can be thought as being related to the Bohr radius of the exciton.
In Figure 5.2 we see how a wave would look inside a bulk solid if the solid has a size much larger than the wavelength. In the case of nanoscale materials, the size of the material becomes smaller than the wavelength and thus the wave has to “squeeze” its wavelength in order to fit within the potential barriers enforced by the nanocrystal surfaces.
By reducing the wavelength we are increasing the frequency of the wave and thus increasing its energy, and this is exactly what happens in nanocrystals. The more the nanocrystal decreases in size, the larger the increase in energy of the excitonic states. The orbitals of the exciton in fact would occupy a much larger space than the nanocrystal, but they are confined by the nanocrystals’ surfaces to fit into a smaller volume. Such an increase in energy of the excitonic states is called quantum confinement.
(To help better understand this a slide is included in digital notes)
What are the effects of quantum confinement?
As shown in Figure 5.2, The electronic denisty of states is profoundly modified by quantum-confinement effects. the smooth and continuous denisty of states of bulk semiconductors is progressively concentrated into distinct peaks, corresponding to the excitonic states.
The Quantum confinement also has the effect that the oscillator strenght* of these excitonic transitions increases dramatically thus making them visible at room temperature.
*The oscillator strenght is a measure of the probability that a photon will be absorbed by a specific electron, leading to an electronic transition.
What are quantum dots (reminder)?
They are semiconductor nanocrystals which can be considered to some degree artificial atoms, where the energy levels can be modified by size and also shape.
Why are quantum dots important?
Read this carefully please:
The control of electronic states in materials allows for the design of materials with specific, tailored properties. For example, the high oscillator strength of certain transitions has enabled the development of highly sensitive near-infrared photodetectors, which have significant applications in areas such as RFID technology, astronomy, and medicine.
Additionally, the strong luminescence of CdSe quantum dots, due to exciton recombination, is being explored for use in highly efficient LEDs that may eventually replace traditional incandescent, fluorescent, and halogen lights. Furthermore, these quantum dots possess a unique ability to generate multiple excitons when exposed to photons with energies that are multiples of the material’s bandgap. This phenomenon, if harnessed, could significantly improve the efficiency of solar cells.
(To help better understand this a slide is included in digital notes)
Why is the control of shape so important for quantum dots?
i) Shape affects the directionality of properties as well as the mutual interaction between nanostructures. For example, the anisotropy of the shape of nanorods has allowed them to be connected in end-to-end chains much like polymer analogues.
ii) To shed light on the relatively unclear mechanisms of nucleation and growth, which are the foundation of modern-day electronic materials, like Si or GaAs.
iii) to control the motion assembly, and bonding of atoms
What is supersaturation (imp)
Supersaturation is the measure by which the concentration of solute in a solvent is higher than its maximum solubility in that solvent.
S = [Cd^2+] [Se^2-] / ksp
where ksp is the solubility product for CdSe, representing the maximum amount of Cd^2+ and Se^2- ions that can be present in water before CdSe starts precipitating out of solution. If the system is supersaturated S will be >1 and CdSe solid should form at equilibrium
How do we synthesize CdSe nanocrystals?
The synthesis of CdSe nanocrystals is generally based on the La Mer model of monodisperse colloid growth, which is shown in Figure 5.2. The principle behind it is fairly intuitive: If you want to produce monodisperse colloids, you will want to nucleate them all at one precise moment, and then rwo them all at the same rate. This is the concept of separation between nucleation and growth, which is at the heart of monodisperse colloid and nanocrystal synthesis.
As we can see in Figure 5.3 the plot of supersaturation versus time of reaction is pretty distinctive. The supersaturation must be quickly increased to yield a burst of nucleation, forming a larger number of stable nuclei. Such a quick nucleation will rapidly consume the reagent, leading to a rapid decrease in supersaturation. In the presence of stable nuclei, further nucleation requires a supersaturation higher than a critical value, which is generally larger than the value obtained after the conclusion of the primary nucleation. That graph thus embodies the separation of nucleation and growth which i the core of La Mer’s model.
